From your question, I assume you are using GNOME Shell. It sounds like this might be due to some settings being cleared, or changed. (I can't remember which it was, but I'm pretty sure I bumped into this a while ago). You can configure your own maximize and unmaximize shortcuts in the Keyboard settings panel. Open System Settings from the menu with your name at the top of the screen, click Keyboard and then choose Shortcuts. Now, under the Windows section, you should see items for "Maximize window" and "Restore window". Click each one you wish to change and enter the shortcut you want.